Lack of "--update" flag in older Ubuntu distros (#56283)

When installing Ansible on older Ubuntu distributions be aware of the lack of ``-u`` or ``--update`` flag.

+label: docsite_pr

docs/docsite/rst/installation_guide/intro_installation.rst

@@ -150,7 +150,7 @@ To configure the PPA on your machine and install ansible run these commands:
     $ sudo apt-add-repository --yes --update ppa:ansible/ansible
     $ sudo apt install ansible
 
-.. note:: On older Ubuntu distributions, "software-properties-common" is called "python-software-properties". You may want to use ``apt-get`` instead of ``apt`` in older versions.
+.. note:: On older Ubuntu distributions, "software-properties-common" is called "python-software-properties". You may want to use ``apt-get`` instead of ``apt`` in older versions. Also, be aware that only newer distributions (i.e. 18.04, 18.10, etc.) have a ``-u`` or ``--update`` flag, so adjust your script accordingly.
 
 Debian/Ubuntu packages can also be built from the source checkout, run: